The present invention relates generally to sensors and, more particularly, to a micromachined sensor with a single anchor and motion-limiting connection geometry.
This invention involves MEMS (micro-electro-mechanical systems) sensors that use a vibratory or flexural element.
MEMS sensors are generally produced by micromachining silicon wafers using photolithographic techniques. The small size of these elements, typically around 1 square millimeter, enables the production of large numbers of devices from a single silicon wafer.
One typical example of a MEMS sensor is a micro-gyro. A micro-gyro measures the angular rate of rotation about an input axis or so-called xe2x80x9crate axisxe2x80x9d. Micro-gyros may generally be classified as linear or as rotary. In either case, a mass is driven into vibration relative to a xe2x80x9cdrive axisxe2x80x9d that is orthogonal to the rate axis. Gyros are designed such that if the mass is subject to rotation relative to the sensor""s rate axis at some angular rate of rotation, then coriolis forces will cause the vibration of a sense element relative to a xe2x80x9csense axisxe2x80x9d that is orthogonal to the rate and drive axes.
The unique micro-gyros developed by the assignee of this invention and disclosed in U.S. Pat. No. 5,955,668, commonly owned by the assignee of this invention and hereby incorporated by reference in its entirety, are statically xe2x80x9cde-coupledxe2x80x9d in that the drive mass and the sense element may move independently. In the absence of a rotational rate input, the oscillation of the drive mass about the drive axis does not induce a substantial motion in the sense element. In the presence of rotational rate, however, a coriolis-induced oscillation about the sense axis is dynamically transferred from the drive mass to the sense element through suitably designed flexures.
In the embodiments described in the ""668 patent, the sense element was anchored to the substrate at two or more points. Other gyro and accelerometer designs typically use multiple anchors, including those described by U.S. Pat. Nos. 5,408,877; 5,515,724; 5,569,852; 5,574,222; 5,578,755; 5,618,989; 5,627,317; 5,631,422; 5,646,347; and 5,408,119. The use of multiple anchors effectively inhibits the sense element from oscillation about the drive axis, but residual stresses from manufacturing processes and thermal stresses due to material expansion or contraction between the anchors may affect the sensor performance. The issues of thermal and manufacturing stresses are common to all varieties of MEMS sensors, not just micro-gyros. Limiting the motion of the supported element, however, is often paramount. It would be advantageous to develop a micromachined sensor that reduces the stresses associated with multiple anchors while providing a motion-limiting connection geometry that substantially restricts the sense element to a single motion, specifically rotary or translational oscillation with respect to a sense axis; in other words, constrains the motion of the sense element to a single degree of freedom. The embodiments described in the ""668 patent were an improvement to the existing art, but further embodiments have been developed which provide this additional advantage.
In a first aspect, the invention resides in an improved micro-electro-mechanical systems (MEMS) sensor having a substrate and a sense element that exhibits a motion relative to the substrate in response to an influence to be sensed, the improvement comprising: a single centrally-located anchor extending from the substrate to support the sense element; and a means for connecting the sense element to the single anchor which substantially constrains the motion of the sense element to a single degree of freedom.
In a second aspect, the invention resides in a MEMS sensor comprising: a substrate; a sense element having a central aperture, said sense element exhibiting a sense motion relative to a sense axis passing through the central aperture in response to an influence to be sensed; a single anchor extending from the substrate within the sense element""s central aperture to support the sense element; and a means for connecting the sense element to the single anchor, the connecting means having a geometry that substantially constrains the motion of the sense element to a single degree of freedom corresponding to the sense motion.
In a third aspect, the invention resides in a MEMS sensor comprising: a substrate; a sense element that is located in a plane substantially parallel to the substrate, that has a central aperture, and that exhibits a motion in response to an influence to be sensed; a single anchor extending in a substantially perpendicular direction from the substrate to within the sense element""s central aperture to support the sense element; and connecting elements that connect the single anchor to the sense element within the central aperture, the connecting elements having a geometry that substantially facilitates a sense motion of the sense element relative to a first sense axis passing through the central aperture and substantially inhibits other motion relative to the first sense axis and motions relative to second and third axes that are perpendicular to the first sense axis and to one another.